I guess this is in response to playback as a) the bird is so close and b) the song is not fully developed. Being late in the breeding season (but a little early for second pairings based on my experience), I would guess that song was not really on this bird's mind and it only sang half-heartedly in response to playback.
The use of playback on birds in TPK is frequent, it seems to me, particularly with HBF. I wish photographers would be more mindful of the impact they have on birds such as this.
Apologies in advance if this recording was made without the use of playback! The principle still applies however.
GeoffC
